GF07.61-P-4030AMG ME-SFI Anti-Knock Control, Function
GF07.61-P-4030AMG ME-SFI Anti-Knock Control, Function
The task of anti-knock control (AKC) is to ensure knock-free operation of the engine with different grades of fuels from idle speed up to the maximum engine speed.
For this purpose, if uncontrolled combustion (knocking) occurs, the ignition angle at the relevant cylinder or cylinders is retarded. The knock control is integrated in the ME-SFI [ME] control unit.
Input signals:
- Knock identification
- Engine speed
- Ignition circuit recognition
- Coolant temperature
- Intake air temperature
- Engine load
- Camshaft position
- Map-based pilot control by the automatic research octane number switch
- Presetting (research octane number correction) with the STAR DIAGNOSIS
Output signals:
- Ignition angle correction for specific cylinder (to the map value calculated by the ME-SFI [ME] control unit)
- Limp-home ignition angle (load value limitation) for a recognized knock sensor error (knock regulation limp-home).
The knock control is active from a coolant temperature of 40 °C. If cylinder knocking is recognized above an engine load of about 40%, its ignition angle is adjusted to retarded by about a 3.0 ° crank angle after the next ignition. If knocking continues to occur the ignition angle is continuously retarded in steps of a 3° crank angle until the maximum retardation is reached of approx. a 10 - 15° crank angle depending on the engine speed.
In addition, the automatic RON switch influences maximum retardation. The higher the RON stage selected, the less the maximum retardation.
For knock-free combustion retarding the ignition timing in operation steps of about a 0.75° crank angle is undertaken after some ignitions rpm dependent until the normal performance map value is reached again or new knocking occurs.
If knock control gives an ignition angle adjustment over a longer period, the ME-SFI [ME] control unit reduces the power to protect the engine.
Adaptation of knock control
The adaptation of the knock control is active as of a coolant temperature of approx. 75 °C. Any corrections required to the ignition angle because of knocking combustion are constantly detected for the specific cylinder and stored together with the relevant operating state of the engine (engine load/speed range).
When determining the individual ignition angles, the stored adaptation values are taken into account, so that the optimal ignition angle can be calculated immediately for each cylinder.
During an abrupt acceleration, no values are stored for about 2 seconds in order to avoid any incorrect adaptation values.
Faults if knocking is detected
If no signals are received from the knock sensors, the ignition angle of all cylinders is retarded by as much as a 11° crank angle for safety reasons, depending on engine speed and coolant temperature. The knock control and the adaptation are blocked and a fault is stored in the fault memory.
Automatic "RON switch"
In addition to RON correction using STAR DIAGNOSIS, the knock control features an automatic RON switch. This detects a persistent high knocking tendency of the engine and assists the knock control by means of an appropriate pre-control.
This is done by analyzing the stored adaptation values, the ignition angles of all the cylinders and the number of control interventions in certain engine load and speed ranges. Depending on the threshold which has been exceeded, the RON value is adjusted by one or several stages. The value is retained until the engine is switched off. After engine start, only the RON correction which is carried out by means of STAR DIAGNOSIS is at first active., component description.